1. Field of the Invention
The invention relates to catalytic converters for purifying exhaust gases, and more particularly to catalytic converters for purifying exhaust gases from a motorcycle internal combustion engine.
2. Description of the Related Art
Automobile and motorcycle exhaust gases are conventionally purified with a catalyst supported on a ceramic body able to withstand high temperatures. The preferred catalyst support structure is a honeycomb configuration that includes a multiplicity of unobstructed parallel channels sized to permit gas flow and bounded by thin ceramic walls. The channels may have any configuration and dimensions provided gases can freely pass through them without being plugged by entrained particulate material. Examples of such preferred structures included the thin-walled ceramic honeycomb structures described in U.S. Pat. No. 3,790,654 to Bagley and in U.S. Pat. No. 3,112,184 to Hollenbach.
Ceramic honeycomb catalyst supports are exposed to high temperatures resulting from contact with hot exhaust gases and from the catalytic oxidation of uncombusted hydrocarbons and carbon monoxide contained in the exhaust gas. In addition, such supports must withstand rapid temperature increases and decreases when the automobile engine is started and stopped or cycled between idle and wide-open throttle. Such operating conditions require the ceramic honeycomb catalyst support to have a high thermal shock resistance, a property generally inversely proportional to the coefficient of thermal expansion.
Ceramic supports for catalytic converters are typically formed from brittle, fireproof materials such as aluminum oxide, silicon oxide, magnesium oxide, zirconium silicate, cordierite, or silicon carbide. The typical honeycomb configuration of supports made from these ceramic materials enables even very small mechanical stresses to cause cracking or crushing. In view of their brittleness, a great effort has been expended to develop catalytic converter housings, or cans, for such supports.
For example, U.S. Pat. No. 4,863,700 to Ten Eyck discloses a catalytic converter system where a frangible ceramic monolith catalyst is resiliently mounted in a metallic housing by an insulating layer of ceramic fibers wrapped around the monolith, and a layer of intumescent material disposed between the metal housing and the ceramic fiber layer.
In many applications, particularly those involving small motorcycle engines, there is little room for mounting catalytic converters. One such solution to this problem is to mount catalytic converter within existing exhaust system components rather than providing an additional catalytic converter housing; one such location being within a hot gas chamber which includes the expansion chambers and mufflers.
A complication of locating the converter inside the muffler housing is that the converter inside the muffler is not allowed cool efficiently enough to maintain standard intumescent mats within a favorable thermal environment (&lt;550.degree.C.); specifically, encapsulation within an insulated hot gas chamber such as a muffler prevents such converters from efficiently dissipating heat to the atmosphere. Furthermore, in such applications, the hot exhaust gas not only flows through the catalytic converters, but also around its housing. Consequently, in such applications the temperature of the catalytic converter housing assembly (i.e. the housing which maintains the converter in its correct position inside the hot gas chamber) commonly approaches 900.degree. C. Furthermore, significant concentrations of gaseous raw fuel and oil typically appear in the exhaust gas stream, with the fuel-rich exhaust producing extreme exotherms within the converter resulting in temperatures as high 1100.degree. C. Standard vermiculite based intumescent mats typically lose their ability to expand if exposed to temperatures greater than .about.750.degree. C. Specifically, intumescent mats lose their chemically-bound water when exposed to such high temperature. The loss of chemically-bound water damages the intumescent character of the material so that it does not provide adequate mounting pressure to retain the ceramic catalyst support. This jeopardizes the ability of the ceramic catalyst to withstand axial and other forces resulting from exhaust gas flow and vehicle vibration. Intumescent mats, therefore, do not offer a viable option for internally mounted motorcycle converters.
Ceramic fiber mats, capable of exposure to temperatures as high as .about.1200.degree. C., represent an alternative to intumescents. The force generated by these mats is developed completely from the compression it undergoes during the canning of the catalytic converter. As such, the form of canning is critical to these fiber-based mats.
Stuff mounting is one method of canning that has been utilized in the past. Initially, the substrate is wrapped with the mat and inserted into a conical device that compresses the mat as it is pushed through. The wrapped substrate is then ejected from the compression cone into a cylindrical tube that serves as the converter shell. In the process of performing this activity, the mat must be maintained within a very narrow dimensional gap between the can and the substrate to be effective; acceptable fiber-based mat gap bulk density (GBD) is typically 0.55.+-.0.05 g/cc. Problems inherent in the stuff mounting method include: (1) a gap which is too large, resulting in insufficient gripping pressure of the substrate and typically slipping of the wrapped substrate during vehicle operation; and (2) an over-compressed mat, resulting in damage to the mat, and ultimately leading to gas erosion.
Additional problems associated with stuff mounting include: (1) variability in the mat basis weight is .about.10% which alone results in some so-formed converters falling outside of the aforementioned acceptable GBD range; (2) substrate diameter variability; and, (3) variability in the metal shell tube diameter, into which the mat/substrate is placed. Even if the tolerance stack-up issues could be tolerated, stuff mounting these fiber based mats, at such high gap bulk densities, is an inefficient process, at best. The mat must be so "over-compressed", in the stuffing cone, prior to being injected into the finished tube, such that some of its 2-dimensional resiliency is lost (due to fiber damage). Furthermore, it has been observed that shear forces acting on the mat has caused some portions between the substrate and the shell to "leak" out of the gap at the top of the stuff mounted part. This loss of some of the mat from the gap, in turn has resulted in lower than desirable compressive forces holding the substrate in place.
As such, there continues to be a need for a catalytic converter that will remain securely mounted inside a hot gas chamber even at operating temperatures exceeding 800.degree. C.